Which of these numbers is composite? ${17,\ 47,\ 85,\ 89,\ 97}$
Solution: A composite number is a number that has more than two factors (including 1 and itself). 17, 47, 89, and 97 each have only two factors. The factors of 85 are 1, 5, 17, and 85. Thus, 85 is the composite number.